The present invention relates generally to an automatic transmission, which is disposed for a speed ratio change in the power transmission path between the engine and the drive wheels of a vehicle. Furthermore, the present invention relates particularly to a control system that controls the automatic transmission for a shift from an off-going speed ratio to an on-coming speed ratio.
Automatic transmissions have been used in vehicles. Generally, an automatic transmission for use in a vehicle comprises a plurality of gear trains, which are disposed parallel with one another, a plurality of frictionally engaging elements such as friction clutches, which are used for selecting a certain gear train for power transmission from these gear trains, and a shift control valve, which controls the actuation of the frictionally engaging elements. For such an automatic transmission, for example, Japanese Laid-Open Patent Publication Nos. 11(1999)-108169 and 11(1999)-201270 disclose a shift control system.
In the shift control performed by the control system, the shift control valve functions to release a clutch which has been in engagement (hereinafter referred to as xe2x80x9coff-going clutchxe2x80x9d or xe2x80x9coff-going frictionally engaging elementxe2x80x9d), and to bring another clutch into engagement (hereinafter referred to as xe2x80x9con-coming clutchxe2x80x9d or xe2x80x9concoming frictionally engaging elementxe2x80x9d) for shifting the gear trains of the transmission, from the off-going speed ratio to the on-coming speed ratio. If the release of the off-going clutch and the engaging actuation of the on-coming clutch are not controlled appropriately, then a shift shock, an engine racing, etc. may happen, damaging the smooth shifting of the gear trains. To avoid such discomforts, these clutches must be controlled in a timely manner. Therefore, a prior-art control system has incorporated, for example, a release-timing control valve which controls the timing for releasing the off-going clutch in synchronization to the increase of the hydraulic pressure used for the actuation of the on-coming clutch (this valve is also referred to as xe2x80x9corifice control valvexe2x80x9d by the applicant of the present invention).
However, this release-timing control valve is designed simply to function in response to a change in the hydraulic pressure which actuates the on-coming clutch. In this control system, the release-timing control valve opens to release the hydraulic pressure of the off-going clutch for disengaging this clutch when the hydraulic pressure actuating the on-coming clutch has increased to a certain level. Because of this limited performance of the release-timing control valve, it is difficult for the control system to respond effectively to all the shifts that are requested and executed in the different conditions of the transmission, for example, to a power-on shift, i.e., a shift executed while the accelerator pedal is pressed down, or to a power-off shift, i.e., a shift executed while the accelerator pedal is released. Depending on the condition where a shift is executed, a shift shock can occur.
For example, whether the shift being executed is a power-on upshift or a power-off upshift, as long as the shift is executed from the same speed ratio, the release-timing control valve functions in an identical way to reduce the hydraulic pressure of the off-going clutch when the hydraulic pressure of the on-coming clutch for engagement has increased to a predetermined value. In this case, the release-timing control valve is designed to make the power-on shifts smooth and not to cause an engine racing. However, for a power-off shift, there is a delay in the response of the release-timing control valve to release the off-going clutch, and this may cause a shift shock.
It is an object of the present invention to provide a control system for a vehicular automatic transmission, which system always performs a smooth shift operation by controlling appropriately the timing for releasing the off-going frictionally engaging element in correspondence, for example, to the accelerator opening (engine throttle opening).
In order to achieve this objective, the present invention provides, for an automatic transmission, which can be used in a vehicle, a control system that comprises a plurality of frictionally engaging elements (for example, the FIRSTxcx9cFOURTH speed clutches 31xcx9c34 described in the following embodiment), which are actuated for engagement upon receiving a hydraulic pressure to establish selectively a plurality of speed ratios. In this control system, an upshift from an off-going speed ratio to an on-coming speed ratio is executed by controlling the release of the hydraulic pressure from the frictionally engaging element used for the off-going speed ratio and by controlling the supply of the hydraulic pressure to the frictionally engaging element used for the on-coming speed ratio. The control system further comprises an off-going pressure releasing valve (for example, the first and second off-going pressure releasing valves 70 and 80 described in the following embodiment), which releases the hydraulic pressure of the frictionally engaging element used for the off-going speed ratio, and release timing adjustment means (for example, the linear solenoid valve 60 described in the following embodiment), which controls the operation of the off-going pressure releasing valve. The release timing adjustment means controls the operation of the off-going pressure releasing valve in correspondence to the throttle opening of the engine. In this case, the smaller the throttle opening of the engine, the earlier the timing for releasing the hydraulic pressure from the frictionally engaging element used for the off-going speed ratio is set.
While an upshift is being executed, if the frictionally engaging element used for the off-going speed ratio is released before the frictionally engaging element used for the on-coming speed ratio engages sufficiently, then an engine racing may happen, damaging the smoothness of the shift operation. This problem is likely to be manifested in a power-on shift. On the other hand, if the release of the frictionally engaging element used for the off-going speed ratio is delayed, then the condition of the transmission will be such that both the frictionally engaging elements for the off-going speed ratio and the on-coming speed ratio are in engagement. This condition results in a shift shock, damaging the smoothness of the shift operation. This problem is likely to be manifested in a power-off shift, in which the torque being transmitted becomes small. To clear such likely problems, the control system according to the present invention adjusts, by the release timing adjustment means, the timing for releasing the hydraulic pressure from the frictionally engaging element used for the off-going speed ratio earlier for a smaller throttle opening of the engine (i.e., the closer to a power-off condition, the earlier the timing). In this way, the control system sets the timing for releasing the frictionally engaging element used for the off-going speed ratio appropriately for either a power-on shift or a power-off shift and always achieves a smooth shift operation.
Also, it is preferable that the release timing adjustment means control the operation of the off-going pressure releasing valve in correspondence to the speed of the vehicle. In this case, the lower the speed of the vehicle, the earlier the timing for releasing the hydraulic pressure from the frictionally engaging element used for the off-going speed ratio is set. While the throttle opening of the engine is kept constant, the lower the speed of the vehicle, more likely a shift shock to occur if the timing for releasing the frictionally engaging element used for the off-going speed ratio is delayed. The release timing control according to the present invention, however, provides an improved smooth shift operation for any speed of the vehicle because the control is executed in correspondence to the speed of the vehicle.
Sometimes, an automatic transmission for use in a vehicle comprises a torque converter with a lock-up mechanism, the torque converter being connected to the output shaft of the engine. In this case, the release timing adjustment means comprises a linear solenoid valve, and a control pressure supplied from the linear solenoid valve is used for controlling the operation of the pressure releasing valve, and also, while no shift is being performed, it is used for controlling the operation of the lock-up mechanism. In this way, the linear solenoid valve provided as the release timing adjustment means can also be used for controlling the operation of the lock-up mechanism while the vehicle is in ordinary travelling condition (i.e., while no shift is being executed). As a result, the control system can be constructed in a simple design.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.